Before discussing the usage differences between “can” and “could,” I’d like to first disabuse your mind from the mistaken idea that there’s no significant difference between the usage of the modals “will” and “would.” Of course, as I explained
in my reply to YoungMentor last week, there’s not only a significant but a big difference in the sense of these two modals. That difference is the direct, straightforward sense of the modal “will” as opposed to the politeness and deferential sense of the modal “would” in conveying intent or desire. Indeed, there’s a big world of difference between saying “
Will you join me in my campaign?” and “
Would you join me in my campaign?” The first is a plain invitation, while the second is a polite or deferential request—and the difference between these two statements says a lot about the social station or rank of the speaker and the person being addressed.
Perhaps what gave you the wrong notion that there’s no significant difference between “will” and “would” was the exceptional case that YoungMentor asked about, where the speaker is instructing someone to do something in a certain way: “The preposition that
would yield the correct sense is, of course, ‘from among,’ meaning that the ‘one or two people’ specified in the sentence belongs to the totality of all the people visiting hospitals.” As I explained in my column, “would” is the preferred usage in this case but “will” is also acceptable. We shouldn’t generalize on this single exception though.
Now to your main question: “Do ‘can’ and ‘could’ have the same nuances as ‘will’ and ‘would’?” Again, I’d say that it’s slippery—even dangerous—to answer that question with either a categorical no or a yes. To begin with, when we use “can” and “could,” we are conveying the idea of ability, possibility, permission, or potential; in contrast, when we use “will” and “would,” we are conveying the idea of desire, choice, willingness, consent, or habitual or customary action. From the semantic standpoint, therefore, “can” and “could” couldn’t possibly have the same nuances as “will” and “would,” for the each of these modal pairs refers to entirely different ideas.
Maybe the only valid comparison we can make between these two modal pairs is that they vary in a grammatically similar way in their past tense forms. “Would” conveys the idea of past habitual action, as in “In his younger days, he
would take long hikes in the countryside,” and also serves as the past tense of “will” in indirect speech introduced by a verb in the past tense, as in “The convicted plunderer promised that he
would not run for public office anymore.” In the same token, “could” conveys the idea of past ability, as in “When Anna was 4 she
could already play to piano,” and also serve as the past tense of “can” in indirect speech introduced by a verb in the past tense, as in “The ventriloquist claimed that he
could make a Boeing 767 vanish from sight.” I guess these parallel grammatical situations are the closest you can get to saying that “can” and “could” have the same nuance as “will” and “would.”
